Precious metals including gold, silver, palladium, platinum, and their mixtures or alloys are used in the electronics industry for the manufacture of thick film paste.
Mixtures of palladium and silver are widely used in conductor compositions for hybrid integrated circuits. They are less expensive than gold compositions, are compatible with most dielectric and resistor systems and are suitable for ultrasonic wire bonding. The addition of palladium to silver greatly enhances the compatibility of the circuit for soldering, raises the melting point of the silver for compatibility with the dielectric firing temperatures and reduces the problems of silver migration which can cause degradation of the dielectric properties and shorting.
Palladium or palladium alloys are used in electrode materials for multilayer ceramic capacitors (MLCs). The properties of the metallic components of thick film inks intended for the internal electrodes of multilayer ceramic capacitors are extremely important because compatibility is required between the metal powder and the organic medium of an ink and between the ink itself and the surrounding dielectric material of the MLC. Pd powders suitable for use in multilayer ceramic capacitors must also be deagglomerated to adequately disperse in the organic medium and low in surface area to minimize low temperature sintering.
Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become more narrow in width with smaller distances between lines. This is especially true where multilayer ceramic capacitors are requiring thinner and narrower electrodes. The metal powders necessary to form dense, closely packed, narrow lines must be as close as possible to monosized, smooth spheres. The conductive metal powders must have a small particle diameter, an even grain size and a uniform composition.
Palladium oxide has not been widely used in electronic applications because of the inability to make smooth, dense, spherical palladium oxide particles.
Many methods currently used to manufacture metal powders can be applied to the production of palladium and palladium oxide powders. Chemical reduction methods, physical processes such as atomization or milling, thermal decomposition and electrochemical processes can be used. Palladium powders used in electronic applications are generally manufactured using chemical precipitation processes.
Palladium salts such as chloropalladous acid or palladium nitrate are used as starting materials for chemically precipitating palladium powder and palladium oxide. Palladium oxide is chemically produced by solution hydrolysis by increasing the pH of an acidic palladium salt solution until the palladium hydroxide is precipitated. This material then is converted to palladium oxide through dehydrolysis and drying. This process is hard to control and tends to give irregular-shaped, agglomerated particles.
Palladium oxide can also be produced through oxidation of palladium powder in air at high temperatures. Powders produced by this method are very non-uniform with low density.
In making palladium powder, a palladium salt is reduced by using reducing agents such as hydrazine, formaldehyde, hyposphorous acid, hydroquinone, sodium borohydride, formic acid and sodium formates. Metal powders prepared by the chemical reduction of simple metal salts tend to be hard to control, vary in surface area, irregular in shape and agglomerated.
The aerosol decomposition process involves the conversion of a precursor solution to a powder. The process involves the generation of droplets, transport of the droplets with a gas into a heated reactor, the removal of the solvent by evaporation, the decomposition of the salt to form a porous solid particle, and then the densification of the particle to give fully dense, spherical pure particles. Conditions are such that there is no interaction of droplet-to-droplet or particle-to-particle and there is no chemical interaction of the droplets or particles with the carrier gas.
The major problem that has limited successful application of the aerosol decomposition process for powder generation is lack of control over particle morphology. In particular, it was the requirement that the material must be treated above its melting point to form fully dense particles and that operation below the melting point tended to give impure, hollow-type particles which were not densified.